Wow, this is case is doing a favor to my imagination. How about these possibilities, some ancient, but same same, since salt preserves:
1. A medieval (or other) heart burial that was inadvertently dug up during salt collection.
2. A religious death ritual that involved sending human body parts down a river, eventually to intermingle with the salt water of the ocean. That water eventually making it's way into salt flats via the ebb and flow of tides. I have seen salt flats like that in Africa. I have not seen a ritual like that, but humans have every which kind of death ritual, at least one I know of where body parts notoriously end up—for whatever reason—in the waters of a VERY large river.
3. A body part originating in a punishment for a criminal event, a kind of tit for tat execution, like an "eye for an eye".
4. Voodoo. Perhaps the heart was deemed spooked, and accordingly had to be removed. That would be Haiti, but some other countries I've been to as well.
5. Leonardo da Vinci et al. dissected LIVE humans to study body parts, musculature, etc. Horrible. These could easily have been disposed of haphazardly.
6. Per the Boston article above, DOT was getting its salt from Chile. Chile "disappeared" untold hundreds of people during the Pinochet era. They could easily have ended up in a salt mine.
7. And then, there's the Arabian peninsula salt-providers, and there are some regimes there that are notoriously brutal. Cutting off body parts is not especially rare. I've no idea where they'd be disposed, but maybe at a salty lake bed.
8. A heart separated out during a human sacrificial event.
9. A cemetery dug up during road building.
10. The Dead Sea?
I am sure I'll have more, but I have to catch up with sources, especially:
Many went missing between 1973 and 1990 during the dictatorship of Augusto Pinochet – and relatives are still seeking answers
www.theguardian.com
en.wikipedia.org
Examining heart extractions in ancient Mesoamerica (University of Chicago Press)